In general, a polymer electrolyte fuel cell generates electricity and heat by electrochemically reacting a fuel gas containing hydrogen and an oxidizer gas containing oxygen. The polymer electrolyte fuel cell is capable of working at a low temperature of 70-80° C. and of maintaining great current density. In these reasons, the polymer electrolyte fuel cell has fast startup performance, can be miniaturized, and can be made into light weight cells, thus suitable for use in such applications as portable power source, power source for vehicles, residential equipments of steam supply and power generation, etc.
FIG. 1 illustrates an embodiment of the polymer electrolyte fuel cell, which comprises a membrane electrode assembly (MEA, 10) comprising a polymer electrolyte membrane and an electrode, a gas diffusion layer (fluid distribution layer, 12) delivering the gas used in a reaction to the electrode and discharging the reaction products, a conductive bipolar plate (separator, 14) supplying a reaction gas and a coolant from outside and separating oxidized electrode (anode) from deoxidized electrode (cathode), and the like. A fuel cell is composed by stacking these membrane electrode assembly, gas diffusion layer and bipolar plate as many as necessary, and the stack forms a single body with an appropriate pressure given from outside by a equipment, so each unit cell is not out of line or slipped.
Also, a number of manifolds (20) are formed in the upper part and the lower part of the membrane electrode assembly (10) and the bipolar plate (14) for supplying or discharging hydrogen, oxygen needed in a reaction, and coolant needed to cool the reaction heat. And hydrogen, oxygen and coolant supplied from outside are taken into the electrode passing through a pipe outside the stack, the manifold of the bipolar plate, and a gas-flow path formed on the bipolar plate of each unit cell.
On the other hand, a sealing means should be included to prevent hydrogen, oxygen and coolant from leaking from each manifold and the reaction site where hydrogen and oxygen react. However, in the fuel cell, often stopping are repeated by its own characteristics, and expansion and contraction are frequently occurred during the fuel cell operation because of the heat generated by the chemical reaction. Therefore, a sealing structure for the fuel cell must exert sealing performance in the case of frequent expansion and contraction, and only if the stress distribution arising in each element of fuel cell in expansion and contraction is as uniform as possible, the fatigue failure can be prevented.
For this, a gasket is disposed around the electrode and manifold. As a gasket for sealing the fuel cell, silicon sheet or Teflon sheet strengthened by glass fibers is often used because of its easy manufacturing advantage and little thickness deviation.
This strengthened silicon sheet or Teflon sheet has an excellent mechanical strength supported by internal glass fibers, so it can exert mechanical toughness under the excessive pressure in the time when a stack is bound. But the rate of contraction and restoration are not so high that, when the fuel cell operates, gas is apprehended to leak for the expansion of parts by heat and water Moreover gas can leak through the surface of the gasket because the surface is rough and the material is relatively hard.
Another defect is that, if formed thicker than a gas diffusion layer when a stack is bound, the resistance increases because the mechanical strength is greater than that of a carbon paper or a carbon cloth generally used as a gas diffusion layer, and the contact between a gas diffusion layer and a bipolar plate is not tight. On the contrary, there is a problem that, if formed too thin, gas leaks because the pressure on the surface of the gasket is not enough. Therefore it is difficult to determine the proper thickness.
Another way for sealing the fuel cell is to use rubber with superior elastic restitution force and soft property containing silicon, fluorine or olefin as a material of a gasket. There are the way of manufacturing a gasket in the shape of O-ring using a metal mold, the way of jet molding with a metal mold being placed directly on a bipolar plate, and the way of manufacturing a gasket using a dispenser, etc in the way of manufacturing a gasket of rubber.
The way of manufacturing a gasket in the shape of O-ring using a metal mold has the defect that, after manufacturing a gasket, it must be placed on the surface of a bipolar plate one by one when a stack is bound. And the way of jet molding with a metal mold being placed directly on a bipolar plate has the defect that, in the time of manufacturing a gasket, the shape and the dimension of a metal mold must be same with the gasket.
Also, the conventional way of manufacturing a gasket using a dispenser is the way of putting sealant in an injector and the like and pressing it, so has the problem that the height of rubber at starting point and ending point cannot be set uniformly. That is, the liquid state of rubber material is filled along a route of a sealing groove using a dispenser operated by X-Y axis robot, after forming the sealing groove in advance on a bipolar plate, with a width and a depth. The rubber overlaps in ending point with that of starting point, so height become greater than that of other part.
Hereby the pressure of the surface on the bipolar plate and the membrane electrode assembly become nonuniform when a stack is bound, so not only sealing performance is lowered but also the life span of fatigue failure is shortened by this nonuniform stress distribution when used in the case of long term repeatedly.